ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-5-3093-20053-D microphysical model studies of Arctic denitrification: comparison with observationsDaviesS.1MannG. W.1CarslawK. S.1ChipperfieldM. P.1KettleboroughJ. A.2SanteeM. L.3OelhafH.4WetzelG.4SasanoY.5SugitaT.51Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, UK2Rutherford Appleton Laboratory, Didcot, Oxford, UK3Jet Propulsion Laboratory, Pasadena, California, USA4Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany5National Institute for Environmental Studies, Ibaraki, Japan1611200551130933109This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is available from http://www.atmos-chem-phys.net/5/3093/2005/acp-5-3093-2005.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/3093/2005/acp-5-3093-2005.pdf

Simulations of Arctic denitrification using a 3-D
chemistry-microphysics transport model are compared with
observations for the winters 1994/95, 1996/97 and 1999/2000. The
model of Denitrification by Lagrangian Particle Sedimentation
(DLAPSE) couples the full chemical scheme of the 3-D chemical
transport model, SLIMCAT, with a nitric acid trihydrate (NAT)
growth and sedimentation
scheme. We use observations from the Microwave Limb Sounder (MLS)
and Improved Limb Atmospheric Sounder (ILAS) satellite
instruments, the balloon-borne Michelsen Interferometer for Passive
Atmospheric Sounding (MIPAS-B), and the in situ NO<sub>y</sub>
instrument on-board the ER-2.
As well as directly comparing model results with observations, we also
assess the extent to which these observations are able to validate the
modelling approach taken. For instance, in 1999/2000 the model captures
the temporal development of denitrification observed by the ER-2 from late
January into March.
However, in this winter the vortex was already highly
denitrified by late January so the observations do not provide a
strong constraint on the modelled rate of denitrification.
The model also reproduces the MLS observations of denitrification in early
February 2000. In 1996/97 the
model captures the timing and magnitude of denitrification as
observed by ILAS, although the lack of observations north of ~67&deg; N in the beginning of February
make it difficult to constrain the actual timing of
onset. The comparison for this winter does not support previous
conclusions that
denitrification must be caused by an ice-mediated process. In 1994/95
the model notably underestimates the magnitude of denitrification
observed during a single balloon flight of the MIPAS-B instrument. Agreement
between model and MLS HNO<sub>3</sub> at 68 hPa
in mid-February 1995 is significantly better.
Sensitivity tests show that a 1.5 K overall decrease in
vortex temperatures, or a factor 4 increase in assumed NAT nucleation
rates, produce the best statistical fit to MLS observations. Both adjustments
would be required to
bring the model into agreement with the MIPAS-B observations.
The agreement between the model and observations suggests that a NAT-only
denitrification scheme (without ice), which was discounted by
previous studies, must now be considered as one mechanism for the
observed Arctic denitrification.
The timing of onset
and the rate of denitrification remain poorly constrained by the available
observations.